This invention relates to a catalyst for the polycondensation step of polyester production. More particularly, this invention relates to a stable catalyst for the production of linear polyesters and co-polyesters. This invention also relates to a polymerization process employing the catalyst.
Polymers and copolymers of alkylene terephthalate have found wide-spread commercial acceptance. For example, polyesters are used in the manufacture of textile fibers, films, resins, etc.
It is known that polyalkylene terephthalates can be prepared from a suitable alkyl ester of terephthalic acid formed by initially reacting the appropriate alcohol with terephthalic acid. When a methyl ester of terephthalic acid is used as a starting material, it is first reacted with alkylene glycol in the presence of a transesterification catalyst by means of an ester interchange reaction. When terephthalic acid, itself, is used as a starting material, it is subjected to a direct esterification reaction with alkylene glycol in the presence of what is generally called the first stage catalyst additive or ether inhibitor. In either method the resulting reaction product, an ester, is then polycondensed in the presence of a polycondensation catalyst to form polyalkylene terephthalate.
To polymerize a bis(hydroxyalkyl) terephthalate in a reasonable time it is necessary to use a catalyst. Many catalysts have been disclosed for this purpose but it has been found that those giving a rapid production rate also tend to bring about a rapid rate of polymer degradation. Another disadvantage is that many of the known catalysts produce a polymer having a yellowish or gray color. For the manufacture of fibers a color as near white as possible is required and for film making a clear bright polymer is necessary.
It has also long been known in the art that trivalent antimony compounds are excellent polycondensation catalysts. Antimony oxide has been long employed for this use as have various salts and alcohol derivatives such as the alkyl and aryl antimonites, the antimony glycolates, antimony acetates, and antimony oxalate.
However, many of these antimony-containing catalysts produce undesirable side effects, such as a slow rate of reaction, or polymer having a gray color which is undesired in the final terephthalate polymer. It is believed, as discussed in U.S. Pat. No. 3,732,182, that poor rates and gray color formation are caused by a reduction of the antimony catalyst to finely divided metallic antimony. This problem as indicated above has long been known and many attempts have been made to provide a solution.
For example, the aforementioned U.S. Pat. No. 3,732,182 proposes the utilization of the reaction product of an .alpha., .beta.-dicarboxylic acid with an antimony compound as a polycondensation catalyst. U.S. Pat. No. 3,822,239 discloses a complex of an acid anhydride, a glycol and an antimony compound for the same application. Another example of a complex containing an acid anhydride is found in U.S. Pat. No. 3,903,120 which teaches the preparation of zerovalent nickel complexes useful as catalysts for the hydrocyanation of olefins, while U.S. Pat. No. 3,272,786 discloses low molecular weight polymers containing a tri-alkylstibine and maleic anhydride in admixture with other monomers.
Now it has been found in accordance with this invention that selected catalysts are efficacious polycondensation catalysts for the preparation of polyalkylene terephthalate.